1. Field of the Invention
This invention relates generally to "in vitro" analysis to accurately measure the resultant ablative profile of corneal tissue to be reshaped by ablation, allowing the capability to compare a measured (i.e., achieved) ablation profile against the intended profile. The present invention further relates to the ability to determine any differences between the achieved ablation profile and the intended ablation profile to provide information allowing adjustment to an ablation laser's operating parameters to correct for any differences. The present invention additionally relates to the creation of a permanent record of an achieved ablation profile.
2. Background of Related Art
Currently refractive laser systems used to ablate human corneal tissue are calibrated utilizing a flat sheet of polymethyl-methacrylate (PMMA) material. In such a system, a refractive correction is ablated onto the surface of the PMMA plate using an ultraviolet (UV) laser beam source, and the achieved refractive power of the ablated area is measured with a lensometer. Ablation profiles achieved in PMMA material have generally been measured utilizing either a lensometer or a surface profiling system, e.g., a surface scanning instrument manufactured by Tencor Instruments, and extrapolated to predict the refractive power that would be achieved from the reshaped corneal surface. The PMMA correction achieved (typically expressed in diopters) is compared to the desired, or target, corneal correction.
Unfortunately, one problem with utilizing PMMA for this type of calibration is the fact that PMMA material when exposed to UV laser radiation has an ablation rate that is approximately 25% of the ablation rate of human corneal tissue. For example, a target ablation of 10 diopters on human corneal tissue results in an actual achieved test ablation of only about 2.5 diopters when performed on the conventional PMMA calibration material.
For this reason, UV ablation of PMMA material can be effectively utilized only for calibration of a refractive laser system: not to measure an actual ablation profile intended for human corneal tissue. For instance, test ablations are performed using a conventional PMMA material to verify standardized laser system performance on a day to day basis or between cases. As an example, a series of test ablations performed in PMMA material utilizing the same system parameters can be a reasonable verification of day to day equivalent system performance when the ablating laser is operated at those exact test parameters.
Unfortunately, since PMMA ablates at a rate equivalent to approximately 25% of the rate on human corneal tissue, this calibration technique is insensitive to slight changes in system parameters, including those which may affect the performance in corneal tissue but which may not be evident in the ablation of a calibration material at a fraction of the ablation rate.
U.S. Pat. No. 5,464,960 to Hall et al., is an example of a laser calibration device which uses PMMA material. According to Hall et al., a laser is aimed at a monolayer thin film of PMMA material spin coated over a crown glass slide. The number of laser pulses needed to remove the coating over an area of the film, signaled by a sudden fluorescence as the laser reaches the underlying glass, is used as a calibration factor. This calibration factor is obtained by dividing the thickness of the monolayer film by the number of ablation laser pulses used to reach the sudden fluorescence. Other calibration factors are obtained for the various areas of the targeted surface until the entire area reaches fluorescence. Any spread between these various calibration factors provide a map of the ablative characteristics of the laser beam across its entire width based on an achieved profile in the PMMA material. However, the ablative characteristics might be significantly different if determined based on an ablation of corneal tissue, which has an ablation rate about four (4) times that of the PMMA calibration material.
There is a need for an improved capability for predicting, measuring and recording an ablation profile achieved in corneal tissue.